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The Whites of Their Eyes: the Evolution of the Distinctive Sclera in Humans

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The Whites of Their Eyes: the Evolution of the Distinctive Sclera in Humans PAGE 20 VOLUME 38, 2008 THE WHITES OF THEIR EYES: THE EVOLUTION OF THE DISTINCTIVE SCLERA IN HUMANS Joanna Bickham Department of Sociology and Anthropology James Madison University - Epsilon of Virginia At some point in human evolution, hominins developed eyes that were morphologically unique in comparison with their close primate relatives. Specifi- cally, the amount of exposed sclera increased and the tan or dark brown pigment found in the sclera of other primates was eliminated. Because visual cues and gaze detection are so important to human behavior, it follows that the unique morphol- ogy of the human eye conferred fitness upon those individuals. Several theories have been proposed to explain these changes as adaptations to particular environ- mental and social selective forces, but very few studies have directly tested these theories. Based on the available data, it seems most likely that the unique morphol- ogy of the human eye initially developed as a result of hominins becoming terres- trial predators and was completed by the need to communicate effectively with other hominins. The theories surrounding research in this area of hominin evolution range from very simple—that the white sclera would have signaled health and thus con- ferred reproductive fitness in the same way that a peacock’s brilliantly colored tail attracts a mate—to complex theories that integrate both biological and cultural phenomena (Tomasello et al. 2007). A commonly espoused idea in the scientific community is that the most parsimonious answer to a question is often correct, but no evidence corroborates the mate attraction theory (Tomasello et al. 2007). Two major studies which will be explored in this paper concern the comparative mor- phology of a wide range of primates, how these differences affect behavior, and what inferences one can make regarding the adaptive reasons behind those differ- ences. Hiromi Kobayashi and Shiro Kohshima conducted a study in 1998 that measured the eyes of 88 species of primates using computer-aided image analysis to determine the differences between species and to examine how these differences correlate with biological and environmental variability (Kobayashi & Kohshima 2001). Michael Tomasello and associates have carried out numerous studies using apes, monkeys, and children that explore issues relating to cognition and commu- nication; this body of work is integrated into the “cooperative eye hypothesis,” which will be explored in the body of this paper (Tomasello et al. 2007:316). LAMBDA ALPHA JOURNAL PAGE 21 Kobayashi and Kohshima measured the width/height ratio of the eye out- line (WHR) and an index of exposed sclera size in the eye outline (SSI) to ana- lyze and compare eye shape differences. A total of 874 individuals representing 88 species were recorded by video camera or analyzed from photographs to col- lect eye shape measurements. Information concerning weight, sitting height, crown-rump length, and habitat type was collected from published sources (Kobayashi & Kohshima 2001), and some walking-height and sitting-height in- formation was collected in the Japan Monkey Center. Eye coloration measure- ments of 92 species were recorded by direct observation of living animals and of prepared eye specimens by two persons working independently. Disagreement between the two observers occurred only 8% of the time, concerning the distinc- tion between the eye colors of 6 species. Sclera coloration was divided into four categories: brown, pale brown, partly white, and white. The researchers collected data on eye movement by directly observing primates eating in cages and hu- mans eating alone in a restaurant and calculated ratios of horizontal scanning to vertical scanning by counting eyeball and head movements (Kobayashi & Koh- shima 2001). Kobayashi and Kohshima found that humans have the largest area of ex- posed sclera and have the most horizontal eye elongation among the primates studied. Both SSI and WHR ratios increased in an order similar to what one would observe by following the branching of a primate phylogenic tree: Prosimii, Ceboidea, Cercopithecoidea, Hominoidea. That is, the most primitive primates had the smallest SSI and WHR ratios. In light of this preliminary evi- dence, the researchers postulated that these differences could “reflect some dif- ference in visual function and/or adaptation to some environmental or physio- logical factor such as the habitat and body size of the species” (Kobayashi & Kohshima 2001:424). To test whether or not relationships between the eye dif- ferences and these factors exist, they compared their eye data to known data re- garding eye function, body size, and ecology (Kobayashi & Kohshima 2001). The researchers found that not enough difference exists between the eye functionality of the studied phylogenic groups to explain the eye shape dispari- ties, but correlations with body size and environment were marked. The re- searchers then ranked the subjects using the aforementioned body size measure- ments, finding that the amount of exposed sclera increased with body size. This correlation was especially noticeable in walking height comparisons; the species with the tallest walking height, in this case the humans, have the largest area of exposed sclera (SSI). In primates, eyeball and/or head movement are especially important for the adjustment of perceived images (Kobayashi & Kohshima 2001). “A larger SSI means a smaller iris relative to the eye outline and probably a greater ability for visual field extension by eyeball movement; in eyes with a large SSI the small iris has a wider space to move within the open eye PAGE 22 VOLUME 38, 2008 outline” (Kobayashi & Kohshima 2001:426). Kobayashi and Kohshima hypothe- sized that the correlation between SSI and body size is an adaptation for extend- ing the visual field by eyeball movement, as opposed to head movement. With- out digressing into an intensive explanation, suffice to say that larger animals save energy by using the eyeball to redirect gaze as opposed to turning the whole head (Shultz 1940). Kobayashi and Kohshima tested this hypothesis by observ- ing 18 species and counting the number of head and eyeball movements in- volved in changing gaze direction (2001). The results supported their hypothesis. Observing that the mean value of WHR is “greatest in terrestrial species, moderate in semi-arboreal species and lowest in arboreal species,” Kobayashi and Kohshima theorized that a “horizontally elongated eye outline is adaptive in extending the visual field horizontally by eye movement, and terrestrial life needs more horizontal scanning than vertical scanning” (2001:428). They tested this hypothesis by again observing primates eating in cages and measuring the time and frequency of horizontal and vertical scanning. They found that the ratio of horizontal to vertical scanning is significantly higher in terrestrial species than in arboreal species. As stated previously, Kobayashi and Kohshima separated 92 species into four categories based on the color of the sclera. Most of the primates observed have a dark brown sclera, while a few had lighter brown sclera or a partially white sclera; humans were the only primates observed to have white sclera with- out any pigmentation. Since pigmentation requires some energy, it follows that if primates have this pigmentation, it must have some adaptive function. A theory advanced by S.S. Duke-Elder (1985), states that the pigmentation may be an anti -glare device. If this were true, then nocturnal species should have white sclera and humans should have colored sclera; neither case occurs. Other researchers have proposed that having dark sclera is adaptive in many nonhuman primates because gaze direction is so important in intraspecific communication (Chance 1962; van Hooff 1962; Andrew 1964; Tomasello et al. 2007). Direct eye contact is construed as threatening behavior in many species of monkeys, thus the inability to perceive whether or not another individual is looking at a dominant individual would reduce violent intraspecific altercations (Kobayashi & Kohshima 2001; Perrett & Mistlin 1990). Kobayashi refers to the research of P.W. Sherman in stating that “colored sclera obscuring gaze direction may serve to deceive natural predators . by making it difficult for predators to know if the prey has them in their gaze” (Kobayashi & Kohshima 2001:431; Sherman 1977). Thus prey might gain a slight advantage in concealing their gaze direction from potential predators. Kobayashi and Kohshima realized that if this “gaze camouflage theory” is a factor, then the color of the exposed sclera should LAMBDA ALPHA JOURNAL PAGE 23 be similar to the color of the iris and/or the face around the eye, making it difficult to “detect the position of [the] iris in the eye outline and/or the eye outline in the face” (Kobayashi & Kohshima 2001:431). Another examination of the sclera data becomes necessary. Kobayashi and Kohshima separated 82 primate species into four categories based on the degree of contrast between the sclera and the iris/face. The four types are: (1) dark sclera and similarly dark face and iris, obscuring both eye outline and iris position; (2) sclera darker than iris but similar to face color, obscuring iris po- sition while allowing eye outline in face to be discerned; (3) sclera darker that iris but similar to face color, obscuring eye outline while allowing iris position in face to be discerned; (4) sclera is paler than face/iris and both eye outline and iris posi- tion are clearly discernable (Kobayashi & Kohshima 2001). The researchers found that “almost all [of the] nonhuman primates species observed (80 out of 81 spe- cies) belonged to Type 1 or Type 2 coloration types” (Kobayashi & Kohshima 2001:433). In these types, the direction of gaze is effectively camouflaged by ob- fuscation of the position of the iris in the eye outline and/or the position of the eye outline in the face because of similarly colored features. The only nonhuman spe- cies that did not belong to Type 1 or Type 2 was the ruffed lemur, which has a clearly discernable iris position but an obscured eye outline; this morphology also effectively camouflages the direction in which the animal is gazing.
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